Many electronic control systems and computing devices require an accurate time. One widely used method to accomplish this is to provide an oscillator or other timing device as a “local clock” and to synchronize the local clock to an external “reference clock”. This process is analogous to checking a wrist watch (the local clock) against a presumed accurate time announced on the radio (the reference clock). Even if the wrist watch itself gains or loses time over an interval, regular updates from the radio provide enough information to accurately estimate the current time from the time shown by the wristwatch. The time on the wristwatch can be relied on between updates because it can only drift from correct time by a small amount between updates and that drift can be quantified. Technical challenges, however, arise in some fields of use as a result of very high precision requirements.
The problem of “holdover” is the problem of maintaining an accurate time even when updates from the reference clock are interrupted for a relatively extended interval. The problem of compromise detection involves detecting frequency changes in the reference clock. Providing a method for holdover and protecting against compromise is important in telecommunications, industrial control, power transmission and distribution, military equipment, and also in a growing number of commercial computing applications. For example, financial trading systems depend on the distribution of accurate time to compute trading latency and find patterns in trading results. These trading systems are thus vulnerable to interruptions in or compromise of GPS signals.